Microwave applicators employed to deliver energy in microwave heating applications are widely known. Often radiating elements are used to couple the energy into the target to be heated. These radiating elements are either probe-like antennas which are placed inside the heating target or waveguide structures that contain the energy within their internal dielectric that interfaces with a surface of the target to pass on the energy.
A number of these surface applicators exist, for example U.S. Pat. No. 4,392,039 describes a dielectric heating applicator having a cylindrical metal body filled with a mass of low loss dielectric material to be used in direct contact with a body to form a resonant element creating a TM01 mode at the frequency of the applied microwave energy and having a dielectric filling of value greater than the target. Another example is US 2008/0314894 which describes a dielectric heating applicator having a cylindrical metal body filled with a low loss dielectric material operating a TM01 mode to be used to launch microwave energy into tissue in direct contact with the applicator.
Many devices exist that couple or feed energy from electrical coaxial structures into waveguide structures however the theory of transverse electric (TE) and transverse magnetic (TM) mode launch techniques has long been established in the literature as fundamental elements of microwave components such as dielectric filled waveguides and dielectric loaded antennas.
Traditionally in any waveguide structure including dielectrically filled waveguides a feed mechanism will include a coupling element (typically a coaxial centre conductor) which is placed into the waveguide to excite a particular electromagnetic mode as supported by the waveguide dimensional properties, an example being U.S. Pat. No. 3,128,467. In addition to the waveguide properties the mode selection depends upon the geometry of the coupling structure, for example an electrically grounded coaxial loop creates a magnetic field that can be used to excite the transverse electric (TE) mode, examples being U.S. Pat. No. 3,942,138 and U.S. Pat. No. 3,128,467 and a radiating coaxial probe creates electric fields that can be used to selectively couple into the transverse magnetic (TM) mode examples being U.S. Pat. No. 4,392,039 and U.S. Pat. No. 3,087,129. Although these techniques are well known there is novelty in the fabrication method to create these TE and TM mode coupling mechanisms by the current method.
In some applications it is desirable to physically reduce the dimensions of a waveguide such as the cross-sectional area. As the operational frequency is related to the physical dimensions this can change the frequency performance of the waveguide, with smaller cross-section typically accommodating modes at higher frequencies. As it is often desired to maintain the same operational mode or frequency a common technique is to load the waveguide with a dielectric filling thus restoring the operational frequency to the original position.
The coupling elements or structures are also placed into the dielectric medium which is often a high dielectric electroceramic material. This necessitates the machining of holes in the ceramic to accept the probes or conductive loops which can be expensive and impractical in high volume manufacture.
Another limitation is that any air gap between a conductor and the dielectric can affect the microwave operating performance unless it has been sufficiently accommodated in the design and controlled in the manufacture by tuning. Air gaps also allow the formation of high order modes and in high power applications can create a source of breakdown causing arcing and burning of the ceramic or electrode. The effect of air-gaps are particularly relevant if the Epsilon Relative (Er) value of the dielectric is much greater than that of the surrounding air (Er 1) such as in high dielectric ceramic Er 10, Er 20 Er 40 etc. In dielectric loaded waveguides the ceramic filling must be manufactured to a high accuracy to ensure a good consistence of contact with the conductive walls or stable dimensions of air filed regions to ensure accurate performance. It is common practice to add tuning elements to negate the effects of the air gaps created by manufacturing tolerances. This tuning involves the placement of capacitive or inductive elements such as metallic or dielectric tuning materials to counteract the effects on performance created by the air regions. This is often a time consuming and skilled process that dramatically increases the cost of the product.
One of the limitations of these types of dielectric filled waveguide launch mechanisms is the operational bandwidth of feed network is typically limited by the length of the coupling probes which operate over specific frequency ranges. This limitation can be improved by using the mismatch between the waveguide and the target to cancel with the mismatch between the feed mechanism and the dielectric loaded waveguide.
Another factor is that coaxial microwave components require a conductive electrical attachment mechanism which usually takes the form of a pin (male) and socket (female). In the case of a dielectric waveguide feed a pin or socket is placed within the dielectric material to facilitate connection to an external coaxial feed. This arrangement is difficult to fabricate as most highly conductive metals are not compatible with the high temperatures present during the ceramic sintering process (typ. 1300° C.+) and would require to be added after the ceramic is fired, at which point the hardened ceramic is difficult and expensive to machine.
Another factor in using pin/socket arrangements is that they possess a finite lifetime of connections becoming worn by repeated friction. This is a particular problem in microwave applications as small dimensional changes can have a detrimental effect upon microwave performance.
It is known to use self biasing pins (“pogo pin” or Z-pin) that can accommodate repeated mating cycles in different technical areas, for example as coaxial connections between a connector and circuit board as described in U.S. Pat. No. 6,822,542 and as a plug component in a coaxial connector as can be found in U.S. Pat. No. 7,922,529.
In medical applications the cost of manufacture and lifetime of a reusable part is of importance and it can be prohibitively expensive to create disposable microwave grade components for this market using traditional manufacture techniques such as bulk machining, drilling etc.
For microwave applicators manufactured using these methods a hole would have to be machined into the ceramic material with a separate metallic part being added to create the assembled component. This often necessitates a separate tuning mechanism or a number of iterations to ensure that a reliable design is achieved. As this involves labour this is an expensive method for mass production.